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Crystalline materials have a definite structure, whereas amorphous ones do not, and therefore only rather general statements can be made about a material which, when hot, is ductile but when cold is brittle, and fractures if there is a sudden change of temperature.
The thermal history of glass is of particular importance, because glass that has been cooled quickly retains an imprint or ‘memory’ of its state at the moment before it was cooled.

In the example of a viscous glass melt which is cooled very slowly from a temperature T1 to a lower temperature T2 energy available for molecular movements is gradually reduced, but (because the rate of cooling is very slow) the network has enough time to readjust itself and become more compact. (In some cases devitrification crystals can form when the glass is cooled too slowly at the liquidus
temperature.) The spaces in the silicate network will close somewhat, and the glass at T2 will be denser than it was at T1 (this is quite a different process from that of thermal contraction, which also brings about a slight increase in density). If the same glass is cooled suddenly from T1 to T2, the viscous glass does not have time for the viscous network to compact, and the glass at T2 has the lower
density which would be characteristic of T1.
For this reason, T1 is known as its fictive temperature, and this demonstrates the slight uncertainty about defining the properties of a glass at any particular temperature. This concept appears again, later in the chapter, under transition point (Tg) - or called glass transition temperature.

These are main mechanical properties of glass:

• Viscosity of molten glass Glass
• Anelasticity
• Thermal expansion
• Transition point (Tg)
• Optical properties
• Density
• Hardness
• Brittleness

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